1. Field of the Invention
This invention relates to a laser-energizable device for heating tissue, having particular utility in angioplasty applications for the recanualization of occluded arteries by removal of intravascular plaque and thrombi therefrom.
2. Description of the Related Art
Occulsive heart disease, involving blockage of important coronary arteries, is a major cause of death among persons in the adult population. Even where death does not occur as a result of such disease, occluded arteries in the extremeties, liver, kidney, and brain nonetheless may lead to severe discomfort, loss of normal activity, and attenuation of the individual's quality of life.
The treatment of patients with occulsive arterial disease has generally been effected by two primary methods: pharmacological treatment for moderate arterial obstructions, and surgical treatments, including arterial bypass surgery and/or percutaneous transluminal angioplasty (PTA), in instances of severe stenosis.
Among the surgical options, bypass surgery is expensive, difficult, and depending on the location of the occluded artery, may involve traumatic procedures having high health risks and associated long recovery periods.
PTA, which has increased in popularity among vascular surgeons, involves the placement of a balloon catheter at the site of the constriction, and the subsequent widening of the arterial passage by inflation of the balloon. The benefits of PTA, as contrasted to bypass surgery, include less traumatic procedures, lower costs, respectable success in opening arterial occlusions, and relatively short patient recovery times. A primary disadvantage of PTA, however, is that the material causing the arterial blockage, e.g., arterial plaque or thrombi, is not removed but only pushed aside, with the possibility of future recurrent occlusions resulting from the continued accretion of plaque and/or thrombi to the displaced occluding deposits.
The utilization of laser irradiation for removal of the aforementioned arterial deposits has been advocated as a potentially effective means for the recanualization of occluded arteries. Such advocacy is based on the contention that intravascular laser irradiation can provide the same short-term benefits as PTA, with the additional advantage that the artery-occluding material is permanently removed.
The methods of laser irradiation which have been proposed include (1) direct laser irradiation, in which intravascular plaque and thrombi are removed through abalation, and (2) indirect laser irradiation in which laser energy is converted to thermal energy through the interaction of a laser beam with an absorptive, thermally conductive device, which when brought into close proximity or contact with plaque or thrombic complexes, dissolves, melts or evaporates the artery-blocking material.
Both of the above-discussed methods of laser irradiation involve the insertion of an optical fiber into the arterial channel, to function as a transmissive element for delivery of laser energy to the treatment site.
U.S. Pat. No. 4,519,390 to K. J. Horn discloses a laser catheter assembly including a unitary connector coupling the laser beam with an optical fiber. The patent, at column 5, line 36 to column 6, line 4, discloses to cool the optical fiber during use, by introducing a coolant such as carton dioxide gas into a port on the connector. From the port, the coolant passes through a bore in the connector surrounding the optical fiber, and flows through a length of catheter tubing associated with the fiber into a collet at the distal end of the fiber. The collet has a plurality of slots to allow flow of the coolant past the exposed tip of the optical fiber, and out of the distal end of the catheter assembly.
Although the catheter assembly of this patent has proven highly effective in use, the necessity of providing a separate source of coolant medium, and passing same through the collet past the exposed tip of the optical fiber at the treatment site, where high temperatures are generated, results in the introduction of the coolant medium to the arterial passage, from which it must be removed during treatment. As indicated, carbon dioxide gas is a preferred coolant, and the necessity for substantially completely removing such introduced gas from the arterial system is readily apparent, since any gas in the bloodstream after treatment may adversely affect the health and recovery prospects of the patient.
U.S. Pat. No. 4,592,353 discloses a medical and surgical contact laser probe for irradiating human tissue, including an optical fiber connected to a source of laser energy at one end and to a probe tip of laser transmissive material at its other end, with means for securing the transmissive material in the front position. A coolant medium, e.g., water or a gas (column 5, lines 1-3), is introduced into a catheter tubing surrounding the optical fiber, and laterally discharged at the distal end of the probe, to cool its tip. This device, then, is subject to the same deficiencies in use as the device of the previously described Horn patent.
U.S. Pat. No. 4,582,057 to D. C. Auth, et al., discloses a probe for heating tissue, comprising a probe body and a heat conductive portion forming an external heat transfer surface for contact with the tissue. A diode is mounted in the probe body in thermal contact with the heat conductive portion, so that electricity flowing through the diode generates heat which then is conducted to the heat transfer surface. The diode has a reverse breakdown voltage which is proportional to its temperature, and a power supply is connected to the diode through a suitable conductor. The power supply includes means for adjusting the power delivered to the diode, responsive to variations in the magnitude of the reverse breakdown voltage of the diode, to control the temperature of the probe. A plurality of water jets, which are circumferentially spaced about the body of the probe, direct water along the probe sidewalls in an axial direction to clean blood from the site to be cauterized.
The device of this patent requires the use of electrical source and supply means in conjunction with the thermoelectrical diode component. The device is comparatively complex, and requires electrical apparatus to be introduced into the patient's body. Such introduction of electrical apparatus may be hazardous, particularly in the treatment of coronary arteries of patients having implanted pacemakers, due to the electromagnetic interference generated by the probe, which may damage or interfere with electrocardial functions. Further, the device requires a water jet system, with the attendant disadvantages noted in connection with the art described hereinabove.
In addition to the devices described in the aforementioned patents, thermal probes have been developed for use in laser-energized optical fiber systems, comprising an ellipsoidal stainless steel probe tip formed with an integral neck portion, the proximal part of which is crimped onto the optical fiber. Such devices employ a longitudinal passage in the tip, radially displaced from its central axis, to accommodate a guide wire, and are commercially available in various sizes, as measured by the cross-sectional diameter of the probe tip (i.e., the minor axis dimension of the ellipsoidal tip), ranging from about 1.5 to 3 millimeters. The crimping application of the probe to the optical fiber in these devices provides a connection which however is susceptible to disengagement when the probe assembly is subjected to significant tensile forces during use. In addition to such susceptibility to disengagement of the tip from the optical fiber, these devices require a relatively high laser energy input intensity to effect the heating operation.
Accordingly, it would be a significant advance in the art to provide a laser-energized device, useful for performing tissue heating treatments such as angioplasty, which is cooled in a simple and efficient manner, without the introduction of any externally supplied coolant medium to the treatment site.
Accordingly, it is an object of the present invention to provide such a laser-energized device, which is conductively cooled and does not require the introduction of an externally supplied coolant medium to the treatment site.
It is another object of the present invention to specifically provide a laser angioplasty probe which is characterized by a high degree of structural integrity, even when subjected to significant tensile and/or compressive forces in the course of effecting treatment with the device.
It is a still further object of the invention to provide a laser-energized probe utilizing an optical fiber, in which laser energy is efficiently converted to thermal energy at a low input laser energy intensity, as compared with existing laser-energized thermal probes.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.